In a mobile wireless communication system, fixed stations (base stations) are arranged with a moving range of a mobile station (a terminal) being assumed. Specifically, by placing a plurality of base stations, areas (cells) in each of which the relevant base station can communicate with terminals are made overlap one another, and the base stations are arranged so that a terminal can communicate with any base station anywhere in the assumed range. In practice, however, restrictions in position of arrangement of the base stations and an area (a skip zone) where the terminal cannot communicate with any base station due to the influence of a shielding such as a building arise. To reduce the skip zone, a relay device that relays wireless communication between the base stations and the terminals is inserted. This relay device is a relay device of an Amplify & Forward type (an AF type), having a function of amplifying and transmitting a received signal.
While the relay device of the AF type has a simplified device structure because no baseband signal process is performed, noise is amplified at a receiving end, and therefore a Signal to Noise Ratio (SNR) of the relayed signal is not higher than an SNR at the the data bit sequence, thereby allowing noise components to be eliminated in the stage of transmission by the relay device. With this, the SNR at a transmitting end of the relay device can be made higher than the SNR at the receiving end.
For IMT-Advanced, 3GPP (3rd Generation Partnership Project), which is a standardization entity for mobile communication, has been pushing forward the standardization of LTE-Advanced (hereinafter abbreviated as LTE-A), which is a standard succeeding LTE (Long Term Evolution). In LTE-A, in order to improve use efficiency of cell average frequencies and use efficiency of cell edge frequencies, an introduction of a relay device of the DF type has been studied.
In 3GPP, the relay device is defined as a node having a wireless backhaul line with a donor base station ([Non-Patent Document 1]). According to Non-Patent Document 1, as a wireless backhaul line, two types, Inband backhaul and Outband backhaul, have been studied. The former ensures backhaul-line-dedicated wireless communication resources by using part of wireless communication resources for use in data communication, and the latter ensures backhaul-line-dedicated wireless communication resources separately from the wireless communication resources for use in data communication. In the latter, it is easier to manage the wireless communication resources. As an extreme example, however, if there is no need to use a backhaul line at all, the wireless communication resources allocated as those dedicated to the backhaul line cannot be diverted for data communication use, and therefore the latter has a property in which frequency use efficiency tends to decrease.
Moreover, when a relay device is introduced, a plurality of routes arise, including a route by which the base station and the terminal directly communicate with each other and a route by which they communicate via the relay device. Here, a routing technology of deciding which route is used for actual communication is disclosed in, for example, Patent Document 1. Furthermore, a routing technology when a plurality of relay devices are present between the base station and the terminal is disclosed in Patent Document 2.
Still further, in 3GPP, as a method of using a relay device, Cooperative Relay is suggested in Non-Patent Document 5. In this method, a data signal transmitted by the base station is decoded and retained by the relay device and, when NAK indicating that a terminal has failed to receive the signal is fed back to the base station, the relay device intercepts this feedback. When the base station transmits a retransmission packet, the relay device also transmits the retransmission packet based on the retaining result. This method is known as a technology capable of reducing the number of times of retransmission of Hybrid ARQ.